1. Field of the Invention
The present invention relates to an electronic endoscope that is capable of displaying an auto-fluorescent image for diagnosing a lesion, such as a cancer, and an observed image generated by light having wavelengths of narrow-band in visible light, in addition to a normal full-color image.
2. Description of the Related Art
In an electronic endoscope, a so called “narrow-band image”, which is generated by light having a narrow wavelength range, can be displayed in addition to a normal full-color image. Capillaries close to the surface of the epithelial layer reflect light having short wavelengths; on the other hand, organs in a deep portion in the epithelial layer reflect light having long wavelengths. Therefore, by arranging a color filter that transmits light having short wavelengths or long wavelengths, the capillaries or the organs in the epithelial layer are clearly displayed as a narrow-band image.
Further, by irradiating light, which has wavelengths in the ultraviolet range or in that vicinity (hereinafter, called “excitation-light”), an image based on fluorescent light (hereinafter, called an “auto-fluorescent image”) can be displayed on a monitor. Tissue in the epithelial layer has a fluorescent substance, which emits fluorescent light (hereinafter, called “auto-fluorescent light”) when the excitation-light is illuminated thereon. An object image is formed on an image sensor by the fluorescent light passing through an objective lens. Since the amount of auto-fluorescent light, which is emitted from a lesion or a piece of abnormal tissue, is weak compared to that emitted from the normal tissue, luminance of the lesion or the area adjacent to the lesion in an auto-fluorescent image is relatively small; thus, the lesion can be easily detected compared with the normal full-color image obtained by white light.
To display the normal image and the narrow-band image, for example, two rotating color filters, each of which has different color elements, are coaxially arranged along a light-path, and the imaging process is performed in accordance with the color sequential method. One color filter has spectral transmitting characteristics corresponding to human vision, and the other color filter has spectral transmitting characteristics such that some discrete narrow-band wavelengths are distributed. When displaying the normal image, the color filters are moved along a direction perpendicular to the light-path such that the one color filter corresponding to the perception is disposed on the light-path, and when displaying the narrow band image, the other color filter is arranged on the light-path. Further, to display the auto-fluorescent image, the other color filter is replaced with a filter for the auto-fluorescent image.